The present invention relates to a data processing system and more particularly to a cache/disk subsystem wherein a storage control unit is disposed in the system intermediate a host processor, a cache store and a plurality of disk drive devices to control the transfer of data between the host processor, cache store and the disk drive devices.
Application Ser. No. 207,152 discloses in detail a data processing system wherein a cache store stores segments of data which are copies of segments of data recorded on disks, or copies of data which must be written on the disk. The arrangement is such that when the host processor issues a command to transfer data between the processor and a disk, a copy of the data transferred is entered into the cache memory. The host processor specifies the disk space at which a commanded operation is to take place by specifying the starting address on the disk and the number of words to be transferred. A given command may specify a data transfer length of one word or many segments of words, each segment containing, typically, 1792 words. Since the storage control unit normally writes into the cache store a copy of the data transferred, it is obvious that for very long data transfers a large portion of any data in the cache store may be replaced with the data being transferred. This is undesirable because, statistically, it has been found that the data transferred during a long data transfer is not likely to be used again soon. Furthermore, statistics show that data transferred during very long data transfers that begin and end on segment boundaries is even less likely to be soon used again than data transferred during long data transfers that do not begin on a segment boundary or do not comprise an integral number of segments of data.